Boost rectifier with half-power rated semiconductor devices

ABSTRACT

A rectifier has two half-controlled bridge rectifiers which are connected in parallel to provide DC power to DC bus lines. Each bridge rectifier receives AC power through inductances such as series inductors or an isolation transformer with a single primary and two secondaries. Each bridge rectifier has a full bridge of diodes, with active switching devices connected in parallel with half of the diodes in the bridge. The switching devices can be controlled to provide unity power factor at the AC input lines, allowing lower rated diodes and switching devices to be used.

FIELD OF THE INVENTION

This invention pertains generally to the field of electrical powerconversion and particularly to controlled rectifiers.

BACKGROUND OF THE INVENTION

AC to DC electrical power rectifiers are used in a wide variety ofapplications, including power supplies for various types of electricalequipment and in motor drives. The simplest and least expensive type ofrectifier uses a full or half bridge of diodes to rectify single-phaseor three-phase AC input power to DC power. While rectifiers formed ofpassive diodes are inexpensive and relatively reliable, such rectifierscan introduce significant harmonic distortion to the AC power system towhich the rectifiers are connected. The total harmonic distortion (THD)introduced by such diode rectifiers may not satisfy current standardsand regulations such as the IEEE 519 standard. To address theselimitations, rectifiers have been developed that include activeswitching devices, such as IGBT transistors, connected in parallel withthe diodes of the bridge (e.g., six switches for a full-bridgethree-phase AC to DC rectifier), with the switches being controlled tooperate to provide close to unity power factor and reduced THD ascompared to passive rectifier bridges. Such active rectifiers alsopermit bidirectional power flow, allowing power from regenerative loads,such as large motors, to be delivered back through the rectifier to theAC power system. A disadvantage of such active rectifiers is that theswitching devices are much more expensive than passive diodes,particularly since each active switch must be capable of handling thefull rated current and voltage of the system.

In addition to full-bridge active rectifiers, half-controlled rectifiershave also been developed in which half of the rectifier bridge is formedof passive diodes and the other half has active switching devices inparallel with the diodes. For a three-phase AC to DC rectifier, suchbridges can be formed of a bridge of six diodes and three activeswitches. Half-controlled three-phase pulse width modulated boostrectifiers thus can potentially be lower in cost as compared to arectifier having a full bridge of active devices. See, C. H. Treviso, etal., “A Three-Phase PWM Boost Rectifier with High Power Factor Operationand an Acceptable Current THD Using Only Three Switches,” Proc. of E.P.E'97, 1997, pp. 2.934-2.939; J. Kikuchi, et al., “Performance Improvementof Half-Controlled Three-Phase PWM Boost Rectifier,” Proc. of IEEE PowerElectronics Specialists Conference, 1999, Vol. 1, January 1999, pp.319-324. Such circuits have also been developed for the purpose ofsupplying multiple isolated DC buses and multiple loads by employing twocomplementary half-controlled circuits. J. Kikuchi, et al.,“Complementary Half-Controlled Three-Phase PWM Boost Rectifier forMulti-DC-Link Application,” Proc. of IEEE Applied Power ElectronicsConference, 2000, Vol. 1, January 2000, pp. 494-500. For an individualhalf-controlled circuit, the cost of the system inherently will bereduced since only three active devices are used as compared to sixdevices for the fully controlled counterpart. However, the price to bepaid is higher THD and the presence of low order even harmonics on boththe AC side and the DC side. Thus, both the system efficiency and theperformance are degraded as compared to the full active bridge. Theoverall THD of the system was found to be improved to some extent byusing a lagging power factor current command in J. Kikuchi, et al, 1999,supra. However, appreciable THD still remained which was far fromsatisfactory given the IEEE 519 standards. In addition, a lagging powerfactor command may not suit all rectifier loads and, more typically, aunity power factor interface is preferred or required. Some of theproblems affecting half-controlled rectifiers, such as the lower orderharmonics from the AC side and on the DC bus, are partially solved bycombining two complementary half-controlled rectifiers and applying acoordinated central control algorithm, as discussed in J. Kikuchi, etal., 2000, supra. By using this approach, the AC side THD was improvedmarkedly, but the DC side lower order even harmonics still existed. Themajor objective was to have an isolated DC bus for each half-controlledcircuit.

In boost rectifiers of the type discussed above, inductors are normallyused in series with the input phases in order to reduce the amplitude ofthe switching frequency harmonics of the system. Because the phasecurrents will contain appreciable lower order harmonics in aconventional three-phase half-controlled rectifier circuit as comparedto a fully controlled rectifier circuit, the inductors will incur extralosses. In addition, such systems may be prone to acoustic noiseproblems.

SUMMARY OF THE INVENTION

The rectifier of the invention is capable of delivering performancesimilar to that of a conventional fully rated 3-phase pulse widthmodulated (PWM) rectifier and, in addition, the ratings of the switchesand diodes can be reduced to half the rated power. For example, thesemiconductor devices need only be rated for half of the rated peakcurrent of the rectifier allowing significantly less expensive devicesto be used. The typical problems of a half-controlled circuit with loworder even harmonics on both the AC and DC side are eliminated. Theacoustic noise and losses across the input side inductors can be furtherreduced by combining the inductances on the same core.

The rectifier of the invention includes AC input lines at which AC poweris received by the rectifier and DC output terminals at which DC poweris provided by the rectifier. A first half-controlled bridge rectifierhas an AC input and a DC output which is connected to the DC outputterminals. The first half-controlled bridge rectifier has a full bridgeof diodes connected between the AC input and DC output and controllableswitching devices connected in parallel with half of the diodes in thebridge. A second half-controlled bridge rectifier has an AC input and aDC output which is also connected to the DC output terminals to provideDC power thereto in parallel with the first half-controlled bridgerectifier. The second half-controlled bridge rectifier has a full bridgeof diodes connected between the AC input and the DC output andcontrollable switching devices connected in parallel with half of thediodes in the bridge. Inductances are connected between the AC inputlines of the rectifier and the AC inputs of the first and secondhalf-controlled bridge rectifiers to provide AC power to the bridgerectifiers through the inductances; and a controller is connected to theswitching devices of the first and second half-controlled bridgerectifiers to control the switching thereof.

Where AC isolation is desired, the inductances connected between the ACinput terminals and the AC inputs of the bridges can comprise atransformer having a primary connected to the AC input lines of therectifier and a first secondary connected to the AC input of the firstbridge rectifier and a second secondary connected to the AC input of thesecond bridge rectifier. The first and second secondaries of thetransformer are preferably complementary and oppositely poled. Thetransformer may be a three-phase transformer having a three-phaseprimary and two three-phase secondaries, with the first and secondhalf-controlled bridges formed of six diodes connected to thetransformer secondaries between pairs of upper and lower diodes, andwith the switching devices in each of the half-controlled bridges areconnected in anti-parallel with the three lower diodes in each bridge ofdiodes. The switching devices can comprise IGBTs connected inanti-parallel with the diodes and having gate inputs connected toreceive gate control signals from the controller.

Where AC isolation is not needed, the inductances can comprise inductorsconnected in series between each of the AC input lines of the rectifierand a junction between each pair of diodes in the first and secondhalf-controlled bridge rectifiers. For three phase operation, there arethree AC input lines to the rectifier to receive three-phase AC power,and each of the AC input lines is connected through an inductor to ajunction between one pair of diodes in the first half-controlled bridgerectifier and a pair of diodes in the second half-controlled bridgerectifier. Six diodes in each of the first and second half-controlledbridge rectifiers are connected in pairs of upper and lower diodes andthe switching devices are connected in anti-parallel with the lowerdiodes in the first half-controlled bridge and are connected inanti-parallel with the upper diodes of each pair in the secondhalf-controlled rectifier bridge. The switching devices may againcomprise IGBTs having gate inputs connected to receive gate controlsignals from the controller.

The controller preferably receives signals corresponding to the inputvoltage across at least two of the input lines of the rectifier, theoutput voltage across the output terminals of the rectifier, and theoutput currents from the first and second half-controlled bridgerectifiers. The controller preferably controls switching devices of thefirst and second half-controlled rectifier bridges for unity powerfactor at the AC input lines of the rectifier, although leading andlagging power factor operation is also possible if desired. Eachrectifier is effectively operated as an active filter such that theswitching devices on one bridge rectifier which are active can becontrolled to cancel harmonics in the other bridge rectifier which isacting as a passive rectifier.

In the transformer isolated rectifier configuration of the invention,both of the half-controlled rectifier circuits may be a common emittertype so that an isolated power supply rectifier for the gate drivers isnot necessary. For the transformerless rectifier configuration, thecommon collector type half-controlled circuit will require an isolatedgate driver circuit.

The rectifiers of the invention, with or without a transformer, areshoot-through safe and do not need to incorporate dead timecompensation. Thus, the gate drive circuit requirements for the presentrectifier are simplified as compared to fully controlled rectifiers.Also, because of the absence of dead time, the rectifier of theinvention will not produce distortion in current waveforms at lighterloads, which is typically the case for the conventional six switchthyristor rectifier.

The efficiency of the present rectifier is higher since the switchinglosses across the device is reduced due to lower switching currentsthough the switches and diodes. In general, losses in the semiconductordevices are reduced by almost 16.5% compared to a regular six switchconfiguration, while the losses in the AC side inductors (a transformer)are increased by roughly 2%, so that the combined loss in the rectifierand inductor (transformer) is less with the present rectifier by almost15%. The present rectifier requires two additional current sensors formeasuring the currents of the two independent half-controlledrectifiers. For a given switching frequency, the isolation transformer(or inductors) of the present rectifier will be 5-6% greater in volumecompared to that of a normal rectifier. However, because the losses arelower in the present rectifier, the switching frequency can be increasedup to 25%, and thus the inductor size can be reduced to a smaller volumethan typically used for a conventional rectifier, if desired.

The rectifier of the invention is rugged and fault tolerant because ofits inherent shoot-through structure, and because the present rectifieris more efficient, heat sink requirements are reduced. Conversely, ifefficiency levels similar to a conventional rectifier are acceptable,the rectifier can be operated at higher switching frequencies which, inturn, reduces the input side filter requirements.

Further objects, features and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic circuit diagram of a rectifier in accordance withthe invention having an AC side isolation transformer.

FIG. 2 is a schematic circuit diagram of another rectifier in accordancewith the invention without an AC side isolation transformer.

FIG. 3 is a block diagram showing an illustrative implementation of acontroller for the rectifiers of FIGS. 1 and 2.

FIG. 4 are phase current and voltage waveforms of individualhalf-controlled rectifiers obtained from a simulation of the rectifiersof FIGS. 1 and 2 with unity power factor current commands. The errors(difference between the reference and actual currents) in phase currentsof the lower half-controlled rectifiers are added with the raw referencecurrents of the complementary upper half-controlled rectifier and thecombined waveforms are shown in the upper trace. These waveforms areused to illustrate the half-power rating capability of the circuits ofFIGS. 1 and 2.

FIG. 5 are phase current waveforms of an individual half-controlledrectifier, the raw reference currents for each phase, the actualreference currents (the summation of errors in lower half-controlledrectifier and the raw reference current waveforms of the upperhalf-controlled rectifiers) for the complementary half-controlledrectifier of FIGS. 1 and 2 with a 30° lagging power factor currentcommand.

FIG. 6 are phase current waveforms, raw reference current waveforms andthe actual reference currents for the complementary half-controlledrectifiers of FIGS. 1 and 2 with a 30° leading power factor currentcommand.

FIG. 7 are voltage and current waveforms from a simulation of therectifiers of FIGS. 1 and 2 at a simulated full load (40 kW).

FIG. 8 are current waveforms through different components at full loadfor a simulation of the rectifiers of FIGS. 1 and 2.

FIG. 9 are waveforms of current through different components during loadtransition from 100% to 25% and vice versa for a simulation of therectifiers of FIGS. 1 and 2.

FIG. 10 are waveforms of currents and voltages from a simulation of therectifiers of FIGS. 1 and 2 during load transition from 100% to 25%.

FIG. 11 are waveforms of the phase currents for simulations of therectifiers of FIGS. 1 and 2 along with their combined current waveformand corresponding FFT when operating at full load.

FIG. 12 are waveforms for a simulation of the rectifier of FIGS. 1 and 2showing load current, DC bus voltage (VDC) and the corresponding FFT,when operating at full load.

FIG. 13 are voltage and current waveforms and a corresponding FFT from asimulation of the rectifier of FIGS. 1 and 2 with 45° lagging powerfactor current command.

FIG. 14 are current and voltage waveforms and the corresponding FFT fora simulation of the rectifier of FIGS. 1 and 2 with 45° leading powerfactor command.

FIG. 15 are waveforms over a cycle for one phase current and ofsemiconductor losses for a simulation of the rectifier of FIGS. 1 and 2.

FIG. 16 is a schematic circuit diagram of a rectifier in accordance withthe invention similar to that of FIG. 2 which includes DC side thyristorswitches to allow regenerative flow of power from the load to the ACpower system.

FIG. 17 are waveforms showing phase currents for a simulation of therectifier of FIG. 16 during regenerating operation.

DETAILED DESCRIPTION OF THE INVENTION

The rectifier of the present invention may be implemented in three-phaseAC systems, and with and without AC side transformer isolation. Forpurposes of illustrating the principles of the invention, a three-phaseboost rectifier in accordance with the invention having AC sidetransformer isolation is shown generally at 25 in FIG. 1, and athree-phase boost rectifier in accordance with the invention without ACside isolation is shown generally at 27 in FIG. 2. The principles ofoperation of the boost rectifiers 25 and 27 are essentially similar. Athree-phase AC source 30 (e.g., AC power mains or a generator) providesthree-phase AC power on rectifier input lines 31, 32 and 33, and each ofthe rectifiers 25 and 27 provide DC output power at rectifier outputterminals 34 connected to DC bus lines 35 and 36 which have a DC buscapacitor 37 connected between them. The output terminals 34 may bediscrete connectors but may also be, for example, conductors connectedintegrally with the conductors of the DC bus lines. For purposes ofillustration, an inductance 39 and a resistance 40 are shown connectedin series across the DC bus lines 35 and 36 to represent a DC load, itbeing understood that the rectifiers 25 and 27 may be used to supply DCpower to any type of DC load, including, e.g., DC motors, DC powersystems, and inverters which convert the DC power to AC power atselected frequencies. A controller 42 is connected by lines 43 and 44 totwo of the input phase lines (for illustration, the lines 32 and 33) toprovide one of the AC side phase voltages to the controller, and thecontroller 42 also receives signals corresponding to the bridge outputcurrents I1 and I2 from current sensors 45. The controller 42 also isconnected by lines 46 and 47 to the DC bus lines 35 and 36 to providethe voltage Vdc across the DC bus lines to the controller. Thecontroller utilizes the input phase voltage and the voltage across theDC bus to provide switch control signals on control lines 48 to gatecontrolled switches within the rectifiers 25 and 27 as described furtherbelow.

In the AC isolated rectifier 25 of FIG. 1, a three-phase transformer 50has a primary 51 connected to the three input phase lines 31, 32 and 33,and the primary 51 is coupled through a core 52 to a first three-phasesecondary 54 and a second three-phase secondary 55. The two secondaries54 and 55 are poled to be complementary to each other as illustrated bythe polarity dots shown in FIG. 1. The first secondary 54 provides ACoutput voltages on three output lines 57 to a first half-controlledrectifier bridge 58 formed of six diodes 60, connected in a full bridgeconfiguration, and three gate controlled switches 62, e.g., insulatedgate bipolar transistors (IGBTs) connected in anti-parallel with thelower three diodes in the bridge. The second secondary 55 provides ACoutput power on three lines 65 to a second complementary half-controlledrectifier bridge 67 formed of six diodes 68 connected in a full bridgeconfiguration and three gate controlled switches 70 (e.g., IGBTs)connected in anti-parallel with the lower three diodes 68 in the bridge.The IGBT switching devices 62 shown in FIG. 1 are connected in commonemitter configuration to the lower DC bus line 36, and the IGBT switches70 are also connected in common emitter configuration via a conductor 72to the lower DC bus line 36. The upper diodes 68 of the second rectifierbridge 67 are connected to conduct in a forward direction to the top DCbus line 35, and the upper diodes 60 in the lower bridge 58 areconnected via a conductor 74 to conduct current in a forward directionto the upper DC bus line 35. Thus, the DC outputs of the bridges 58 and67 are connected in parallel with each other across the output terminals34 to the DC bus lines 35 and 36. For purposes of explaining theoperation of the invention, the lower diodes 60 in the first rectifierbridge 58 are labeled D12, D14 and D16, the upper diodes 60 are labeledD11, D13 and D15, and the switches 62 are labeled SW12, SW14 and SW16.The lower diodes in the second bridge rectifier 67 are labeled D21, D23and D25, the upper diodes are labeled D22, D24 and D26, and the switches70 are labeled SW21, SW23 and SW25. The output control lines 48 from thecontroller 42 are connected to the gate inputs of the switching devices62 and 70. The output lines 57 from the first secondary 54 are connectedbetween upper and lower pairs of diodes 60 in the first rectifier bridge58, and the output lines 65 from the second secondary are connectedbetween upper and lower pairs of diodes 68 in the second rectifierbridge 67.

In the non-isolated boost rectifier 27, the AC input lines 31, 32 and 33are connected through inductors 80 to a first complementary bridgerectifier 82, and through inductors 84 to a second complementary bridgerectifier 85. The inductors 80 and 84 are shown in FIG. 2 as woundtogether on common cores 87, which is a preferred and convenient way ofwinding two inductors using a single core, but it is understood that theinductors 80 and 84 may be physically separate and not coupled with eachother. AC power is provided through the inductors 80 on lines 89 to thefirst bridge rectifier 82. The bridge rectifier 82 includes six diodes90 connected in a full bridge configuration, with one of the input lines89 connected between each of the upper and lower pairs of diodes 90, andwith three gate controlled switching devices 92 (e.g., IGBTs) connectedin anti-parallel with the lower set of three diodes 90 that areconnected to the lower DC bus line 36. The upper diodes 90 are connectedvia a conductor 94 to the upper DC bus line 35. To prevent circulatingcurrents, a diode 95 is connected to the lower diodes 90 in the currentpath to the rectifier 82 to prevent current backflow from the rectifier82 to the rectifier 85. The second complementary half-controlledrectifier bridge 85 is formed of six diodes 98 connected in a fullbridge configuration, with input lines 100 connected to provide AC powerto the bridge rectifier through the inductors 84, with one of theconductors 100 connected between each of the upper and lower seriesconnected pairs of diodes 98 in the bridge rectifier 85. Gate controlledswitching devices 102 (e.g., IGBTs) are connected in anti-parallel witheach of the upper diodes in the rectifier 85. The IGBTs 102 areconnected in a common collector configuration through a diode 104(connected to prevent current backflow) to the output terminal 34 andthence to the upper DC bus line 35, and the lower set of diodes 98 inthe upper rectifier are connected via a conductor 105 to the lower DCbus line 36. Thus, the bridge rectifiers 82 and 85 have their DC outputsconnected in parallel across the output terminals 34 and the DC buslines 35 and 36. The diode 104 and the diode 95 prevent circulatingcurrents between the two parallel connected rectifier bridges 82 and 85.

For purposes of illustrating the operation of the boost rectifier 27 ofFIG. 2, the upper three diodes 90 in the first bridge rectifier 82 arelabeled D11, D13 and D15, the lower diodes 90 are labeled D12, D14 andD16, and the switching devices 92 are labeled SWl2, SW14 and SW16. Inthe upper or second rectifier bridge 85, the upper diodes 98 are labeledD21, D23 and D25, the lower diodes are labeled D22, D24 and D26, and theswitching devices 102 are labeled SW21, SW23 and SW25.

FIG. 3 is a block diagram of an embodiment of a controller 42 that maybe utilized for the control of the switching devices in the boostrectifiers 25 and 27. The operation of the controller will be discussedin further detail below. In the exemplary controller 42 shown in FIG. 3,a voltage reference V_(dcref) (the desired DC bus voltage) is providedto a summing junction 110 which also receives the actual measured DC busvoltage V_(dc) from the lines 46 and 47. The difference between thereference voltage V_(dcref) and V_(dc) is provided to aproportional-integral (PI) controller 112, the output of which isprovided to a multiplier 114 which also receives an AC voltage referencefrom a vector reference generator 116 that receives the AC input voltagewaveform from the lines 43 and 44. The output I* from the multiplier 114is provided to control loops that control the switching of the switchingdevices in the two complementary bridge rectifiers 58 and 67 or in thetwo complementary bridge rectifiers 82 and 85. The I2 measured currentis provided through a low pass filter 118, the output of which isprovided to a junction 119 that receives the reference I* from themultiplier 114, and the output of the junction 119 is the I2error whichis provided to a summing junction 121 that also receives the referenceI*. The output of the summing junction 121 is passed through a limiter122 and thence to a summing junction 124 which also receives themeasured current I1. The output of the junction 124 is provided througha hysteresis controller 126 to a gate driver circuit 128 which generatesthe control signals on the lines 48 to switch the switching devices inthe first bridge rectifiers 58 or 82. Similarly, the measured current I2is provided through a low pass filter 130 to a junction 131 where it issubtracted from the reference I*, with the output of the junction 131,I1 error, provided to a junction 134 to be summed with I*. The output ofthe junction 134 is passed through a limiter 135 and thence to a summingjunction 136 which subtracts the measured current I2. The output of thejunction 136 is passed through a hysteresis controller 138 to a gatedriver circuit 140 which provides the gate control signals on the lines48 to the switching devices in the second bridge rectifier 67 or 85. Thehysteresis controllers 126 and 138 are conventional elements and may beimplemented in a conventional manner. An example of a commerciallyavailable hysteresis controller is the TPS5211 High FrequencyProgrammable Hysteretic Regulation Controller from Texas Instruments.

In a boost rectifier, inductors are normally used in series with the ACinput phases in order to reduce the amplitude of the switching frequencyharmonics of the system. In case of the complementary rectifierconfiguration of the present invention, two separate sets of lineinductances are needed. Since the phase currents will containappreciable lower order harmonics in a 3-phase half-controlled circuit,the inductors will incur extra losses. Also the system may be prone tohave acoustic noise problems. These issues may be considered aspotential drawbacks for the half-controlled configuration. However, thepresent invention enables a nearly sinusoidal current waveform to beachieved by combining the currents of the two half-controlled rectifiercircuits. In such a situation it is advantageous to combine theinductances for the two half-controlled rectifier circuits through acommon core. For an isolated version, the inductances can be combinedwith the line side isolation transformer 50 as shown in FIG. 1. For thenon-isolated rectifier as shown in FIG. 2, the windings forming theinductances 80 and 84 can be wound on the same cores 87. By combiningboth of the AC side inductances on the same core, and given that theircombined current waveform is nearly sinusoidal, the core loss and thecore vibrations due to low order harmonics will be nearly eliminated.

It may be seen in FIG. 1 that two similar common emitter typehalf-controlled 3-phase rectifiers 58 and 67 are connected in parallelto the same DC bus lines 35 and 36 to supply a common RL load. On the ACside, the half-controlled bridge circuits 58 and 67 are connected toreceive AC power from two separate complementary secondary windings 54and 55 of the 3-phase isolation transformer 50. The line sideinductances for the boost rectifier 25 are combined as part of theleakages of the secondary windings 54 and 55 of the transformer. Theprimary side 51 of the transformer 50 is connected via the input lines31-33 to a balanced 3-phase AC source 30 (e.g., a generator or AC powermains). The AC side complementary isolation transformer 50 eliminatesthe possibility of circulating current between the two half-controlledrectifiers 58 and 67. The elimination of circulating current between thetwo half-controlled circuits is necessary for the half power ratingoperation of the devices as well as for efficient energy transfer fromthe AC side to the DC bus lines 35 and 36.

In the rectifier 27 of FIG. 2, two complementary type half-controlledbridge rectifier circuits (one common emitter type 82 and another commoncollector type 85) are connected in parallel via the output terminals 34to the same DC bus lines 35 and 36 and to a common RL load. However, itmay be seen that in the rectifier 27, no isolation transformer is usedon the AC side. Instead the inductors 80 and 84 for the half-controlledrectifiers 82 and 85 are combined on the same cores 87. Also, to ensureelimination of the circulating current between the upper and lowerhalf-controlled rectifiers, the diodes 95 and 104 are connected inseries between the rectifier bridges 82 and 85 and before the parallelconnection of the two bridges to the DC bus lines 35 and 36. If thesediodes are not used, during some operating modes power will circulatebetween two half-controlled rectifiers instead of being converted fromthe AC to the DC side. This will unnecessarily burden the ratings of thediodes and switching devices and will increase the losses in therectifier.

In each of the half-controlled 3-phase rectifier bridges 58, 67, 82 and85, only three active switching devices (e.g., IGBTs) are used eventhough the number of diodes used is still six. A total of 6 switchingdevices and 12 diodes are needed for the rectifiers 25 and 27 comparedto a conventional rectifier with 6 switches and 6 diodes. In addition,in the topology of the rectifier 27 of FIG. 2, two DC side diodes 95 and104 are included so that the total number of diodes used is 14. Due tothe bulky nature of the input transformer for rectifier applications, itis often preferred not to have a transformer, and in such applicationsthe additional diodes 95 and 104 constitute a small fraction of the costand bulk of the transformer that is not needed for the circuit of FIG.2. It is understood that reference herein to diode or switching devicein the singular can include multiple series or parallel connected diodesor switches which are combined to provide higher current or voltagerating (or both) than are available from a single device, where desired.

Although the component count of the systems of FIGS. 1 and 2 hasincreased because the two half-controlled rectifier bridge circuits areconnected in parallel to the same load through a common DC bus, thetotal load will be shared equally by both of bridges. Hence, for a 1 pu(per unit) system load each of the two complementary bridges will shareonly 0.5 pu. Thus, all six active devices (58 and 67 in FIGS. 1 and 82and 85 in FIG. 2) need only be nominally rated for 0.5 pu power in therectifiers 25 and 27. The power rating of the switches is thus reducedby 50% compared to a full bridge controlled rectifier where all sixswitches must be rated for 1 pu power. Also, the rating of the diodeswill be reduced to 50% of the rated power of the combined system. As aresult, the combined rating of the transistors in each half-controlledrectifier circuit can be only half the total rated power of the system,allowing much less expensive transistors and diodes to be used. Devicecosts may be expected to scale roughly in direct proportion to powerrating. Thus, the 6 transistors required for the rectifiers 25 and 27 ofthe invention may be expected to be about half the cost of the 6transistors of a conventional fully controlled rectifier. Although morediodes are needed for the rectifiers of the present invention, theratings of these diodes will be lower (and the diodes thus lessexpensive) and the cost of diodes is generally much less than the costof a switching transistor. In a conventional six switch rectifier, allof the diodes used are fast acting diodes. The rectifier 75 withisolation needs six switches and 12 diodes which are rated for halfpower. Only six of these diodes are fast acting diodes and the other sixare line commutated diodes. Typically, the line commutated diodes aremuch less costly than fast acting diodes. Similarly, the non-isolatedrectifier 27 has 6 switches and 14 diodes which are all half powerrated. Only six diodes are fast acting and the other 8 are linecommutated type.

The combination of two half-controlled rectifiers as shown in FIGS. 1and 2 provides improved performance compared to a single half-controlledcircuit. Furthermore, the rectifiers of the invention can achieveperformance comparable to that of a conventional six switch activerectifier by using a cross-coupled control that provides nearlysinusoidal unity power factor interface with the main AC power system.

It can be seen in FIG. 1 that by employing a complementary configurationin the secondaries of the transformer 50, both of the half-controlledrectifier circuits 58 and 67 are achieved with common emitter switchingdevices. By doing so, the requirement for an isolated power supply forthe IGBT gate drivers is eliminated. In addition, it may be seen fromFIGS. 1 and 2 that the half-controlled rectifier circuits areshoot-through protected since each switching device is connected inseries with a diode (and not another switching device), which is animportant advantage of the rectifiers of the present invention over aconventional six switch active rectifier (in which two switches areconnected in series across the DC bus lines). Because of thisshoot-through protection, the rectifier of the invention does notrequire dead time delay, which further simplifies the gate drive circuitand also provides an important performance improvement compared to aregular six switch rectifier. The performance of a normal six switchrectifier is degraded appreciably at lower load due to the dead timethat must be incorporated into its gate driver circuit. With therectifier of the invention, this possibility is eliminated inherently.

As noted above, the efficiency of the rectifier of the invention isimproved as compared to a conventional 6 switch rectifier, since theswitches now carry only 50% of the rated peak current and thus theswitching losses can be reduced considerably. To give a comparison, adetailed loss model of the semiconductor devices has been implemented.With the same semiconductor device loss model, the losses in the presentrectifier and the losses of a conventional six switch rectifier aresimulated dynamically and are compared as discussed further below.

A typical simulation result of a 3-phase half-controlled rectifier 25 or27 as in FIG. 1 or 2 for a unity power factor current command is givenin the lower trace of FIG. 4. It can be seen from the figure that theoperation of the rectifier can be divided into four operating modes. Inmode 1 operation, all the three-phase currents Ia1, Ib1 and Ic1 (on thelines 57 or 89) are well controlled and the phase currents are followingtheir sinusoidal reference currents. In this mode, two switches sw14 andsw12 (see FIG. 1 and FIG. 2, lower half-controlled rectifier 58 and 82)are regulated to control the current in their respective phases. In thiscase, the third phase current, which happens to be the phase B in thiscase, is also being regulated close to its reference value. If oneconsiders the origin of this plot as 0 degrees, then mode 1 operation isspanned from 0-60 degrees as given in FIG. 4 until the current throughphase C passes through zero.

At the 60 degree phase angle, the system enters mode 2 operation whereboth phase B and phase C references demand negative currents, whereasonly sw14 is being regulated. At the same time it can be seen that bothupper and lower half diodes corresponding to phase C (i.e. D15 and D12)are reverse biased and hence cannot conduct.

Thus, the current through this phase (ic1) will continue to be zerountil the corresponding diode is forward biased. In this mode only thephase A switch (sw14) is being regulated and the current through phase Bprovides the return current path. Hence phase B will follow the samecurrent waveform with reverse polarity as phase A during this mode. Thismode of operation continues for the next 300 interval (i.e. 60°-90°)until diode D12 is forward biased. During this mode of operation it maybe observed that both phase B and phase C currents deviate from theirreferences, with deviations of the currents in both the phasesmonotonically increasing until they reach their maximum (i.e. 50% of theindividual rectifier peak current) at the 90° phase angle. Also, it maybe observed that at this phase angle, both the phase B and phase Creference currents are the same and their magnitude is −0.5*Ipk, whichcan be readily calculated to be Ipk*sin(120+90)° and Ipk*sin(240+90)°respectively. From the figure it can be observed that at 90°, phase B isactually carrying 50% more negative current than its reference valuewhereas phase C has a deficit of 50% negative current at the 90° point.

In the lower trace of FIG. 4, the period during 90°-120° is divided intotwo modes: mode 3 and mode 4. In mode 3, phase B and phase C encountercommutation overlap. The current through phase C builds up whereas thecurrent through phase B decreases. The overlap period continues untilthe current through phase B reduces to zero. In a manner similar to aregular 3-phase diode bridge or SCR bridge, the overlap period heredepends upon the line inductance value and the magnitude of current atthe start of commutation. After phase B current reaches zero, the systementers mode 4 operation where the current through phase B will remain atzero and phase C will experience a similar magnitude of current as phaseA but with negative polarity. This mode of operation will continue until120°, at which mode 1 operation will again be entered and the cycle willbe repeated.

It can be noted that at the start of mode 3 the errors in phase B andphase C currents monotonically decrease, and at the beginning of mode 4the polarities of the errors change their directions and the differencesbetween the actual currents and reference currents of phase B and phaseC are always well below the maximum deviation (50% of the individualrectifier peak current) as observed earlier during mode 2 operation.

From the above discussion, it can be concluded that the maximum errorsin phase currents with unity power factor current command is limited to50% of the rated peak value of each half-controlled rectifier. Also, itcan be observed that when only phase A current is being activelyregulated, the errors in currents through phase B and phase C are alwaysopposite in nature. In addition, the errors in phase B and phase C willbe of equal magnitude in the absolute sense for the reasons discussedbelow.

Since the sum of the currents in all three phases of an individualrectifier is always zero and only phase A current is being activelyregulated, the sum of the currents through phase B and phase C will beequal and opposite to the magnitude of phase A current. Hence, if thereis any error in current through phase B that error has to be offset by asimilar error by phase C and the errors must be of opposite sign inorder to preserve Kirchoff's current law. Hence, in order to tracksinusoidal reference currents in phases B and C, an external circuit isneeded which will be capable of subtracting a maximum 50% negativecurrent in phase B and supplying a maximum of 50% negative current inphase C. For this purpose, the second complementary half-controlledbridge 67 or 85 as shown in the upper half of FIGS. 1 and 2 can beemployed.

The operation of the complementary rectifier bridge 67 or 85 is the sameas the operation of the rectifier bridges 58 and 82 except that thecurrent waveforms are controlled sinusoidally during their negativehalves. Thus, during mode 2 operation as explained above, both phase Band phase C currents in the complementary half-controlled rectifier 67or 85 will be controlled by regulating their respective switches (sw23and sw25) and the currents through these phases will follow theirreferences. By observing FIG. 4 it can be inferred that phase B andphase C will carry exactly half of the negative peak currents at the 90°point. Thus, both the phases will have the potential to carry additional50% current without exceeding the current limit of their respectiveswitches.

It was explained above that the maximum errors in the lower half phase Band phase C currents (i.e. ib1 and ic1) during this mode are alsolimited to 50% of the peak current. Therefore, if one adds up the errorsin phase currents of the lower half-controlled rectifier to therespective phase currents of the complementary upper half-controlledrectifier, then the combined waveforms will assume the waveshapes givenin the upper trace of FIG. 4. It may be seen that the magnitude of thecombined waveforms remains restricted within an envelope of 40 A peak topeak current, which is considered as the peak rated current of anindividual half-controlled rectifier for the present analysis. Hence, ifone maintains regulated phase currents in the complementary upper halfconfiguration as given in the upper trace of FIG. 4, effectively theharmonics drawn by the lower half rectifier 58 or 82 will be “cleanedup” satisfactorily without exceeding the current limits for the upperhalf complementary rectifier 67 or 85.

If it is assumed that the lower or first half-controlled rectifier 58 or82 is operating either in mode 2, 3 or 4, it can be found from FIG. 4that the upper or second half-controlled rectifier 67 or 85 will operatein mode 1 during this interval because of their complementary nature.Hence, in the lower half rectifier only the phase A switch, SW14, ismodulated; on the other hand, for the upper half-controlled rectifierboth phase B and phase C switches (SW23, SW25) are modulated. Thus,during this mode the upper half-controlled rectifier will follow thereference current set for it for all the three phases, whereas the lowerhalf-controlled rectifier will encounter errors in phase B and phase Ccurrents. As explained above, the errors in individual phases neverexceed more than 50% of the individual peak limit and the errors inphase B and phase C are always almost equal and opposite in nature.Hence, the sum of the errors will always be close to zero. Since theerrors in phase current of the first half-controlled rectifier will becompensated by the complementary second half-controlled rectifier, theadditional currents in phase B and phase C through the secondhalf-controlled rectifier will be limited to 50% of the individual ratedpeak value. Because the summation of all three phase currents of eachindividual rectifier is zero, the phase A of the upper (second)half-controlled rectifier will also carry an additional currentequivalent to the sum of additional currents in phase B and phase C.Also, since the direction of errors in phase B and phase C currents areopposite in nature and their sum is close to zero, it is very likelythat the phase A current will also remain within the individual ratedpeak value (i.e. 50% of the rated rectifier peak current). Thus, it canbe seen that in the rectifier of the present invention, the currentsthrough all the switches as well the diodes need to be rated for only50% of the rated value of the rectifier as a whole.

By employing a similar method the ripple due to the upper (second)half-controlled rectifier can be “cleaned” by the lower (first)half-controlled rectifier without overshooting the current limits of theupper half-controlled rectifier. Hence, a unity power factor interfaceis possible in accordance with the invention by employing twohalf-controlled rectifiers in complementary fashion satisfying thecondition that currents through individual rectifiers be limited totheir peak rated currents, i.e., half of the combined peak current ofthe upper and lower half rectifiers. Thus, with the rectifier topologyof the present invention, unity power factor interfacing is possiblewith six half power rated active devices and twelve half power rateddiodes.

The rectifier 25 or 27 allows the use of half power rated devices whenunity power factor current commands are used. If a system demands eitherlagging or leading power factor current commands, then the half powerrating active devices will either not support such operation or it maybe supported with degraded performance. This phenomenon can be explainedwith the help of FIGS. 5 and 6 where the current references for therectifiers 25 and 27 is shown for 30 degree lagging and 30 degreeleading power factor current commands operation. In both cases, thecurrent waveforms for a typical half-controlled configuration are givenin the lower traces. The error in phase currents (reference—actual) arethen added with the currents of a complementary half-controlledconfiguration and the resulting waveforms are plotted in the uppertraces. The process is same as explained for the case of a unity powerfactor current command as in FIG. 4. It can be seen from both FIGS. 5and 6 that the upper trace waveforms exceed the limits of the 40 Aenvelope. This fact implies that the switch currents in thecomplementary configuration are required to exceed the assumed 40 Alimit for satisfactory performance. Thus, half power rated activedevices are not sufficient for lagging or leading power factor currentscommands. Also, it is evident from the lower traces in FIGS. 5 and 6that some of the operating modes explained for unity power factorcommands in FIG. 4 do not exist in these cases.

Estimation of losses in any power electronic circuit is an importantconsideration. For proper functioning of the semiconductors and goodpackaging, thermal design plays an important role. The losses incurredin different semiconductor modules and power electronics components needto be dissipated efficiently so that the junction temperature of thedevices does not cross a certain threshold. Therefore, for properthermal design, detailed loss modeling of the semiconductor devices isdesirable.

In general, the device manufacturer supplies device loss characteristicsfor different operating conditions such as during Turn-on, Turn-off, andthe on-state voltages during conduction. The characteristics are complexin nature and depend on parameters such as junction temperature,operating DC bus voltage, current through the device, etc. The losscharacteristics for SKM300 GB123D IGBTs have been fitted with a suitablecurve using MATLAB, and the coefficients of fitted curves are used toobtain a complete loss model for the rectifier. Based on the abovefit-curve of losses and on-state voltages of IGBTs and diodes, adetailed loss model has been developed using the SABER MAST MODEL and isincluded in the dynamic simulation of the system. During dynamicoperation, the DC bus voltage, instantaneous phase currents and theswitching logics are considered as the input for the loss model. Basedon the switching logic status, the occurring of transients likeswitching on, switching off, or the status of different switchingcomponents of the rectifier are decided. The turn on and turn off lossesof an IGBT and turn off losses of diodes are found as a function ofcurrent and dc bus voltage from the fit curve. Similarly, theinstantaneous on-state voltages of the diodes and IGBTs are also foundas a function of current through them. The on-state voltages are thenmultiplied by current, yielding the instantaneous conduction loss. Theselosses when integrated over a specified time result in the totalconduction loss. Summation of turn on loss, turn off loss, andconduction loss of each of the elements gives the total loss of a phaseleg of the rectifier. Similarly, summing up the losses in all phase legsprovides the total loss of the rectifier. The detailed loss modelingused can be found in Bernet et al., “A Matrix Converter Using ReverseBlocking NPT-IGBTs and optimized pulse patterns,” IEEE Power ElectronicsSpecialists Conference, June 1996, Vol. 1, pp. 107-113.

The following discusses the control strategy for the complementarybridge rectifiers 25 and 27 with common DC bus control with reference toFIG. 3. Since both of the half-controlled rectifier circuits 58, 67 and82, 85 are connected to a common DC bus 35, only one DC bus voltagesensor is sufficient (in contrast to a multiple DC bus linkapplication). In the control scheme of FIG. 3, the outer loop isregarded as DC bus voltage control loop. The PI controller 112 is usedfor regulating the DC bus voltage. The output of the PI controllerdecides the amplitude of the current through phases. This amplitude,when multiplied at 114 with sinusoidal unit vectors derived in phasewith the phase voltage, gives the combined current references I* foreach phase of the primary side of the transformer. However, thereference current for each complementary configuration is one-half ofthe combined reference currents. Thus, the combined reference currentsmultiplied by 0.5 gives the reference currents for the phases of theindividual half-controlled rectifier bridges.

In the half-controlled rectifier circuits 58, 67 and 82, 85, the currentof each phase can follow the references only through one-half cycle,whereas the other half cycle current remains uncontrolled and thereforecan not always follow the reference. In such a case, the complementaryside rectifier bridge that is in its controlled half cycle cancompensate for the error due to its counterpart rectifier bridge whichis uncontrolled. In the controller of FIG. 3, an active filter typecontrol configuration is utilized with the current references generatedfrom the voltage controller. It can be seen from FIG. 3 that theindividual rectifier reference currents I* are added at 121 and 134 tothe error of the complementary half-controlled rectifier (I1 error orI2error) to generate the final current references for the individualhalf-controlled rectifiers. The current references through each phase ofboth the rectifiers are limited to half of the peak rated current by thelimiters 122 and 135. The current controllers used are conventionalhysteresis current controllers 126 and 138. Depending upon the directionof the errors between the reference current and actual feedback currentsthrough the phases, the switching of the devices can be executed. Lowpass filters 118 and 130 are used in the feedback path of the measuredcurrents I1 and I2 in order to avoid switching frequency harmonicsaffecting the dynamics of the controller.

The operation and control strategy for both the isolated andnon-isolated type half power rating rectifiers 25 and 27 shown in FIGS.1 and 2 are identical. In both cases, the current control is realizedwith a hysteresis controller 126 and 138. A PI controller generallycannot be used for the current regulator because the half-controlledcircuits are only controlled in one half of the cycle, and hence a PIcontroller would become saturated.

A 40 kW 3-phase 3-wire 415V, 60 Hz system with a 700V DC bus has beensimulated in SABER along with an RL load. Logic BJTs and power diodesavailable from SABER templates were used for simulating the rectifierpower circuit. An inductance of 1 mH in the AC side is used for eachphase of each half of the rectifiers. For the isolated type rectifier,the inductance is included as the leakage inductance of the transformer.A 3300 μF DC bus capacitor with an ESR of 0.015 ohms is considered inthe above simulation. The resistance and inductance used for full loadin the DC side are 12.25 ohms and 100 nH respectively. Since theoperating principle and control algorithms for both the circuits 25 and27 (FIG. 1 and FIG. 2, respectively) are exactly the same, theperformance and the waveforms in both the circuit are expected to be thesame and the results shown are applicable for both topologies.

The results of the simulation in steady-state operation and duringtransient operation of the rectifier with appropriate legends arepresented in FIGS. 7-12. It can be seen from the simulation results thateven though the currents through the individual half-controlledrectifier circuits (ia1 and ia2 in FIGS. 7, 10 and 11) are distorted,their combined waveforms (ia, ib and ic) are smooth and nearlysinusoidal. Also, the combined currents are in phase with theirrespective source voltages (see FIGS. 7, 10 and 11). Thus, unity powerfactor interface of the system is achievable at any load value from noload to full load. The DC bus voltage V_(dc) is also very smooth anddoes not show appreciable distortion (see FIGS. 7, 10 and 12). Hence, itis apparent that the controller successfully eliminates the lower orderharmonics on both the AC side and the DC side of the rectifier.

The currents through the different semiconductor components for both theupper and lower half rectifier of phase A at rated load are shown inFIG. 8. The trace demonstrates that the currents through the switchesi(sw 14), i(sw24) and through diodes i(d 12), i(d24) are switched innature and are limited to only 40 A, which is one-half of the peak phasecurrent 80 A when the system operates at full load (40 kW). Hence, therectifier successfully reduces the rating of the semiconductor devicesto 50% of the rated value without sacrificing the performance of thesystem. In order to compare the legends in the plots, reference shouldbe made to the schematic of the rectifiers 25 and 27 in FIGS. 1 and 2.

The lower half anti-parallel diodes (i(d14) and i(d24)) of eachhalf-controlled rectifier also show almost the same peak current (40 A)as that of the switches sw12, sw24. However, it may be observed (seeFIGS. 8 and 9) that these anti-parallel diodes conduct for one halfcycle and their currents are zero for the other half cycle. Thus, thesediodes do not experience switching current, and it may be expected thatthe switching (turn-off) losses of these diodes are almost zero, so thatthe total switching loss of these diodes and the switches are expectedto be less with the present rectifier than in conventional six switchcontrolled rectifiers.

The rectifier 25, 27 is simulated for a DC side load transition from100% to 25% load, and vice versa, and the corresponding simulationresults are given in FIGS. 9 and 10. It may be seen that theconfiguration works well both in steady-state and during transients, andthe current waveforms on the primary side are always sinusoidal andmaintain unity power factor. At the same time, the current through thesemiconductor components (switches as well as the diodes) remain within40 A, which is 50% of the rated current.

In order to show the rectifier in comparison to a multiple DC bustopology, the results of individual phase currents (ia1, ia2), theircombined current waveform (ia) and the DC bus voltage along with theirFFTs are given in FIG. 11 and FIG. 12, respectively, for a full loadcondition. It can be observed that the combined current waveform as wellas the DC bus voltage are almost free from lower order harmonics eventhough the individual phase currents (ia1, ia2) have considerable lowerorder even harmonics.

The rectifier was also simulated for 45 degree lagging and 45 degreeleading power factor current commands and the results are plotted inFIGS. 13 and 14 respectively. It may be seen that the current waveformsare distorted. However, this is not considered a disadvantage since inmost cases unity power factor operation is desired.

A detailed loss modeling of the IGBTs and diodes and the procedure forloss calculation has been explained above. Based on this method, thelosses occurring for different semiconductor components as well as thecumulative loss of the rectifier have been simulated dynamically and thesimulation results are given for one full cycle in FIG. 15. Lossesacross each component as well as the total loss incurred in one leg of ahalf-controlled rectifier are given in this figure. The‘cummulative_total_loss’ is the cumulative summation of all thecomponent losses of one individual rectifier phase leg shown in theabove figure. It was determined that the total cumulative loss of aphase leg of a half-controlled rectifier over one full cycle ofoperation is 2.22 J. Thus, the total loss over one cycle (16.66 ms) forthe rectifier will be 6 times (for six legs combining twohalf-controlled rectifiers) 2.22 J, which is equal to 13.32 J. The totalloss of the rectifier over 1 second can be calculated as (13.32/0.01666)watt, which is equal to 799.5 watts. The simulation results of thelosses were conducted at the rated power condition with 4 A (i.e. ±2 A)peak to peak hysteresis band for the individual current regulator.

With similar operating conditions (8 A peak-to-peak hystersis band, 1 mHline side inductance and with full load) the losses in a conventionalsix switch rectifier were also simulated and total cumulative loss overone cycle was estimated to be 2.584 J. Thus, the total loss of thisconventional rectifier over one cycle will be 6 times (for six switchesand their anti-parallel diodes) 2.584 J, which is equal to 15.504 J. Thesame can be converted for 1 second as (15.504/0.01666) or 930 watts,which is roughly 16% higher loss than the rectifier of the invention.This example is provided for general comparison purposes only, and isnot an indication that the loss of the rectifier of the invention willalways be better than or of the same magnitude of improvement withrespect to all fully controlled active rectifiers.

A further issue is the effect on the passive elements of the presentrectifier in comparison to the conventional six switch rectifier. Tofacilitate this comparison, the currents through the individualhalf-controlled rectifiers and their FFTs (fast Fourier transformations)are plotted in FIG. 11. From the FFTs it may be calculated, usingstandard practice, that the individual phase currents of each rectifier(ia1 and ia2) have roughly 47% THD with respect to their fundamentals(considering up to the 14th harmonic for the calculation). Also, fromFIG. 7 it may be seen that the rms currents of individual rectifierphase currents (ia1 and ia2) are 29.188 A and 29.614 A respectively,whereas the rms current for their combined waveform (ia) is 56.863 A.This result implies that the total rms current in the secondary windingsof the transformer has been increased by 1.9397 A (29.188+29.614-55.6),which is about 3.4% higher than that of the conventional rectifier.

Also, the resistances of the individual phases of the secondary windingof the present rectifier will be increased by a factor of two assumingthe number of turns remains the same and the area of the cross sectionof each winding is reduced in half. It can be calculated that the copperlosses in the individual secondary winding of the transformer will behigher by around 6.94%(((29.188)²*2R+(29.614)²*2R−56.853²*R)/(56.863²*R))). However, thecopper loss of the primary side remains the same, since the currents onthe primary side still remain sinusoidal with magnitudes at the ratedvalue. Therefore, the total copper loss of the transformer (combiningthe primary and the secondary) with the present rectifier will beincreased by only about 3.5%. The core loss of the transformer isexpected to remain the same as that of a regular six switch rectifiersince the flux through the transformer core is almost sinusoidal withouthaving any appreciable harmonics. Thus, the total loss (combining copperloss and iron loss) of the transformer may increase by around 2% for thepresent rectifier. It can also be inferred that the volume of the ACside isolation transformer for the present rectifier needs to beslightly greater due to the higher secondary rms current requirement andto the separate insulation requirement for the two secondary windings.However, this increase in volume is quite negligible.

Following similar reasoning, for the rectifier configuration 27 in FIG.2, the inductors 80 and 84 will have slightly more volume (5-6% higher)than the inductors in a conventional six switch rectifier. However,since the switching losses in the present rectifier are 16% less and thedead time requirement is eliminated, the switching frequency can beincreased by around 25% using the same switching devices withoutexceeding the thermal limit of the devices. Thus, the inductor size canpotentially be reduced by 25% should this reduction be desirable.

The total ripple current content of a conventional six switch rectifierand the rectifier of the present invention for a similar load and thesame switching frequently are approximately the same. Thus, the DC sidecapacitance requirements for the present rectifier will remainessentially the same as that of a conventional six switch rectifier.

To provide the capability of transferring power from regenerative loadsto the AC power system 30, the non-isolated rectifier 27 of FIG. 2 maybe modified as shown in FIG. 16 by adding controlled switching devices150 and 151 in parallel with the diodes 95 and 104, respectively. Therectifier configuration of FIG. 16 is capable of 100% powerrectification and 50% power regeneration. Preferred switching devices150 and 151 are line commutated thyristors (SCRs) anti-parallel to thediodes 95 and 104. When the rectifiers 82 and 84 are operating inrectification mode, the gate pulses for both the thyristors 150 and 151are turned off by the controller 42 and the circuit acts as a regularrectifier as explained above. However, during regenerating mode ofoperation (such as when a motor load delivers power back to the DC bus),the gate pulses to the switching devices 150 and 151 are kept oncontinuously by the controller 42, the half controlled bridges 82 and 85together act as one regular inverter, and the control of the switchingdevices in the bridges 82 and 85 reverts to a control for a single fullbridge inverter operation. During the regenerating mode, the regularrectification control algorithm remains in standby mode and becomesactive when the rectification mode again comes into operation. Asillustrated in FIG. 3, the output of the PI controller 112 may beprovided to a comparator 160, the output of which is provided directlyand via an inverter amplifier 161 to a gate drive circuit for thethyristor gates so that the thyristors are off during normalrectification and on during ordinary regenerative mode operation. Theregeneration capability of the system is restricted to only 50% of therated power of the rectifier since the switches in the bridges 82 and 85are only capable of handling 50% of the rated current. The performanceof the circuit during steady-state regeneration is illustrated in FIG.17. It can be seen that each half-controlled circuit phase (ia1, ia2)carries currents through only one half cycle, which is the typical casefor any regular inverter. Also, it may be seen that the combined currentwaveform is out of phase with the supply voltage, which confirms theregeneration operation. The peak current through individual phases ofeach half-controlled rectifier is 40 A, which is the same as the peakcurrent of the total phase current (ia). The currents through differentswitch components are also illustrated in FIG. 17. The transition fromrectification mode to regeneration mode and vice versa is smooth and thecurrent waveform is always sinusoidal, and the unity power factorinterface with the supply is maintained at all times.

It is understood that the invention is not confined to the particularembodiments set forth herein as illustrative, but embraces all suchforms thereof as come within the scope of the following claims.

1. A rectifier comprising: (a) AC input lines at which AC power isreceived by the rectifier and DC output terminals at which DC power isprovided by the rectifier; (b) a first half-controlled bridge rectifierhaving an AC input and a DC output connected to the DC output terminals,the first half-controlled bridge rectifier having a full bridge ofdiodes connected between the AC input and DC output and controllableswitching devices connected in parallel with half of the diodes in thebridge; (c) a second half-controlled bridge rectifier having an AC inputand a DC output connected to the DC output terminals to provide DC powerthereto in parallel with the first half-controlled bridge rectifier, thesecond half-controlled bridge rectifier having a full bridge of diodesconnected between the AC input and the DC output and controllableswitching devices connected in parallel with half of the diodes in thebridge; (d) inductances connected between the AC input lines of therectifier and the AC inputs of the first and second half-controlledbridge rectifiers to provide AC power to the bridge rectifiers throughthe inductances; and (e) a controller connected to the switching devicesof the first and second half-controlled bridge rectifiers to control theswitching thereof.
 2. The rectifier of claim 1 wherein the inductancesconnected between the AC input terminals and the AC inputs of thebridges comprises a transformer having a primary connected to the ACinput lines of the rectifier and a first secondary connected to the ACinput of the first bridge rectifier and a second secondary connected tothe AC input of the second bridge rectifier.
 3. The rectifier of claim 2wherein the first and second secondaries of the transformer arecomplementary and oppositely poled.
 4. The rectifier of claim 3 whereinthe transformer is a three-phase transformer having a three-phaseprimary and wherein the first and second secondaries are three-phasesecondaries, and wherein the first and second half-controlled bridgesare formed of six diodes connected to the transformer secondariesbetween pairs of upper and lower diodes, and wherein the switchingdevices in each of the half-controlled bridges are connected inanti-parallel with the three lower diodes in each bridge of diodes. 5.The rectifier of claim 4 wherein the switching devices comprise IGBTsconnected in anti-parallel with the diodes and having gate inputsconnected to receive gate control signals from the controller.
 6. Therectifier of claim 5 further including a DC bus capacitor connectedacross the output terminals of the rectifier.
 7. The rectifier of claim5 wherein the controller receives signals corresponding to the inputvoltage across at least two of the input lines of the rectifier, theoutput voltage across the output terminals of the rectifier, and theoutput currents from the first and second half-controlled bridgerectifiers, and wherein the controller controls the switching devices ofthe first and second half-controlled bridges for unity power factor atthe AC input lines of the rectifier.
 8. The rectifier of claim 1 whereinthe inductances comprise inductors connected in series between each ofthe AC input lines of the rectifier and a junction between each pair ofdiodes in the first and second half-controlled bridge rectifiers.
 9. Therectifier of claim 8 wherein there are three AC input lines to therectifier to receive three-phase AC power and wherein each of the ACinput lines is connected through an inductor to a junction between onepair of diodes in the first half-controlled bridge rectifier and a pairof diodes in the second half-controlled bridge rectifier, wherein thereare six diodes in each of the first and second half-controlled bridgerectifiers connected in pairs of upper and lower diodes and wherein theswitching devices are connected in anti-parallel with the lower diodesin the first half-controlled bridge and are connected in anti-parallelwith the upper diodes of each pair in the second half-controlledrectifier bridge.
 10. The rectifier of claim 9 wherein the switchingdevices comprise IGBTs having gate inputs connected to receive gatecontrol signals from the controller.
 11. The rectifier of claim 9further including a DC bus capacitor connected across the outputterminals of the rectifier.
 12. The rectifier of claim 9 wherein thecontroller receives signals corresponding to the input voltage across atleast two of the input lines of the rectifier, the output voltage acrossthe output terminals of the rectifier, and the output currents from thefirst and second half-controlled bridge rectifiers, and wherein thecontroller controls the switching devices of the first and secondhalf-controlled rectifier bridges for unity power factor at the AC inputlines of the rectifier.
 13. The rectifier of claim 9 further including adiode connected to the first rectifier bridge to prevent backflow ofcurrent from the first rectifier bridge and a diode connected to thesecond rectifier bridge to prevent backflow of current from the secondrectifier bridge.
 14. The rectifier of claim 13 including a thyristorconnected in anti-parallel with the diodes connected to the first andsecond rectifier bridges that can be triggered to conduct duringregenerative operation.
 15. The rectifier of claim 9 wherein the twoinductors connected to each AC input line are wound on a common core.16. A rectifier comprising: (a) three-phase AC input lines at which ACpower is received by the rectifier and DC output terminals at which DCpower is provided by the rectifier; (b) a three-phase transformer havinga primary connected to the AC input lines and a first three-phasesecondary and a second three-phase secondary, wherein the first andsecond secondaries are complementary and oppositely poled; (c) a firsthalf-controlled bridge rectifier having a DC output connected to the DCoutput terminals, the first half-controlled bridge rectifier having afull bridge of six diodes connected to the first secondary between pairsof upper and lower diodes, and three controllable switching devicesconnected in parallel with three lower diodes in the bridge; (d) asecond half-controlled bridge rectifier having a DC output connected tothe DC output terminals to provide DC power thereto in parallel with thefirst half-controlled bridge rectifier, the second half-controlledbridge rectifier having a full bridge of six diodes connected to thesecond secondary between pairs of upper and lower diodes, and threecontrollable switching devices connected in parallel with three lowerdiodes in the bridge; and (e) a controller connected to the switchingdevices of the first and second half-controlled bridge rectifiers tocontrol the switching thereof.
 17. The rectifier of claim 16 wherein theswitching devices comprise IGBTs connected in anti-parallel with thediodes and having gate inputs connected to receive gate control signalsfrom the controller.
 18. The rectifier of claim 16 further including aDC bus capacitor connected across the output terminals of the rectifier.19. The rectifier of claim 16 wherein the controller receives signalscorresponding to the input voltage across at least two of the inputlines of the rectifier, the output voltage across the output terminalsof the rectifier, and the output currents from the first and secondhalf-controlled bridge rectifiers, and wherein the controller controlsthe switching devices of the first and second half-controlled bridgesfor unity power factor at the AC input lines of the rectifier.
 20. Arectifier comprising: (a) three-phase AC input lines at which AC poweris received by the rectifier and DC output terminals at which DC poweris provided by the rectifier; (b) a first half-controlled bridgerectifier connected to the DC output terminals, the firsthalf-controlled bridge rectifier having a full bridge of six diodesconnected in upper and lower pairs of diodes to the DC output terminalsand three controllable switching devices connected in parallel withthree lower diodes in the bridge; (c) a second half-controlled bridgerectifier connected to the DC output terminals to provide DC powerthereto in parallel with the first half-controlled bridge rectifier, thesecond half-controlled bridge rectifier having a full bridge of sixdiodes connected in upper and lower pairs of diodes to the DC outputterminals and three controllable switching devices connected in parallelwith three upper diodes in the bridge; (d) inductors connected in seriesbetween each of the AC input lines of the rectifier and a junctionbetween each pair of upper and lower diodes of the first and secondhalf-controlled bridge rectifiers to provide AC power to the bridgerectifiers through the inductors such that there are two inductorsconnected to each input line; and (e) a controller connected to theswitching devices of the first and second half-controlled bridgerectifiers to control the switching thereof.
 21. The rectifier of claim20 wherein the switching devices comprise IGBTs connected inanti-parallel with the diodes and having gate inputs connected toreceive gate control signals from the controller.
 22. The rectifier ofclaim 20 further including a DC bus capacitor connected across theoutput terminals of the rectifier.
 23. The rectifier of claim 20 whereinthe controller receives signals corresponding to the input voltageacross at least two of the input lines of the rectifier, the outputvoltage across the output terminals of the rectifier, and the outputcurrents from the first and second half-controlled bridge rectifiers,and wherein the controller controls the switching devices of the firstand second half-controlled bridges for unity power factor at the ACinput lines of the rectifier.
 24. The rectifier of claim 20 furtherincluding a diode connected to the first rectifier bridge to preventbackflow of current from the first rectifier bridge and a diodeconnected to the second rectifier bridge to prevent backflow of currentfrom the second rectifier bridge.
 25. The rectifier of claim 24including a thyristor connected in anti-parallel with each of the diodesconnected to the first and second rectifier bridges that can betriggered to conduct during regenerative operation.
 26. The rectifier ofclaim 20 wherein the two inductors connected to each AC input line arewound on a common core.
 27. A method of controlled rectification of ACpower to DC power comprising: (a) providing a rectifier comprising: (1)AC input lines at which AC power is received by the rectifier and DCoutput terminals at which DC power is provided by the rectifier; (2) afirst half-controlled bridge rectifier having an AC input and a DCoutput connected to the DC output terminals, the first half-controlledbridge rectifier having a full bridge of diodes connected between the ACinput and DC output and controllable switching devices connected inparallel with half of the diodes in the bridge; (3) a secondhalf-controlled bridge rectifier having an AC input and a DC outputconnected to the DC output terminals to provide DC power thereto inparallel with the first half-controlled bridge rectifier, the secondhalf-controlled bridge rectifier having a full bridge of diodesconnected between the AC input and the DC output and controllableswitching devices connected in parallel with half of the diodes in thebridge; and (4) inductances connected between the AC input lines of therectifier and the AC inputs of the first and second half-controlledbridge rectifiers to provide AC power to the bridge rectifiers throughthe inductances; and (b) controlling the switching of the switchingdevices of the first and second half-controlled bridge rectifiers toprovide a selected power factor at the AC input lines.
 28. The method ofclaim 27 wherein the switching of the switching devices is controlled toprovide unity power factor.